1. Field of the Invention
This invention relates generally to beta-type free-piston Stirling cycle engines and coolers coupled to a linear alternator or linear motor and more particularly relates to balancing such a coupled system to minimize vibration without the need for a passive vibration balancing unit as is conventionally used.
2. Description of the Related Art
Stirling cycle engines are recognized as efficient thermo-mechanical devices for transducing heat energy to mechanical energy for driving a mechanical load. Similarly, Stirling cycle coolers are recognized as being efficient for transducing mechanical energy to the pumping of heat energy from a cooler temperature to a warmer temperature, making them useful for cooling thermal loads including to cryogenic temperatures. These engines and coolers, collectively known as Stirling machines, are often mechanically linked to a linear motor or linear alternator. A Stirling engine may drive a linear alternator for electrical power generation and a Stirling cooler may be driven by a linear motor. Linear motors and alternators have the same basic components, most typically a permanent magnet that reciprocates within a coil wound on a low reluctance ferromagnetic core to form a stator, and are therefore collectively referred to herein as a linear electro-magnetic-mechanical transducer.
Although a Stirling machine can be linked to a linear electro-magnetic-mechanical transducer in a variety of configurations, one of the most practical, efficient and compact configurations uses the beta-type Stirling machine having its linked linear electro-magnetic-mechanical transducer integrally formed with the Stirling machine and all contained within a hermetically sealed casing. In this configuration, all the reciprocating components reciprocate along a common axis of reciprocation. These reciprocating parts include a piston, a displacer, any connecting rods, the reciprocating magnets and mounting or support structures.
The reciprocating motion of these parts causes oscillating forces to be applied to the casing which results in vibration of the casing and any object to which the casing is mounted. In order to reduce, minimize or eliminate this vibration, the prior art mechanically links an externally or internally mounted vibration balancer, sometimes misnamed a vibration absorber, to the casing. The vibration balancer, most typically a passive vibration balancer, increases the cost and volume of, and adds substantial weight to, the combined and linked Stirling machine and linear electro-magnetic-mechanical transducer. The vibration balancer typically must be tuned with very high precision to the actual operating frequency and this is often difficult. Additionally, the effectiveness of the vibration balancer deteriorates if the operating frequency of the coupled Stirling machine and linear alternator or motor drifts away from the resonant frequency to which the vibration balancer is tuned. A vibration balancer can also cause unwanted dynamic behavior of a Stirling cooler by causing the cooler to have an engine mode operating in conjunction with the normal cooling mode resulting from the generation of beat frequencies.
Therefore, it would be desirable, and is an object and feature of the invention, to provide for vibration balancing of a beta-type Stirling machine coupled to a linear electro-magnetic-mechanical transducer in a manner that eliminates the need for a vibration balancer and reduces the weight and the precision tuning requirements and yet adds only a few additional components of minimal mass and volume to the coupled machines, thereby also reducing cost. This also results in improved specific power for electrical power generation and improved specific capacity for coolers.
FIG. 1 illustrates a beta-type Stirling machine 10 coupled to a linear electro-magnetic-mechanical transducer 12 and having a vibration balancer all according to the prior art. The beta Stirling machine 10 has a power piston 14 that reciprocates within the same cylinder 16 as that in which a displacer 18 also reciprocates. The displacer 18 is fixed to a connecting rod 20 which extends into connection to a planar spring 22. The power piston 14 sealingly slides on the connecting rod 20 and is connected to a second planar spring 24.
The power piston 14 carries a circumferentially arranged series of permanent magnets 26 which reciprocate with the power piston 14. The magnets 26 reciprocate between the pole pieces of a low reluctance core 28 with an armature winding 32 wound on the core 28 to form a stator 30. The stator 30 with its armature winding 32 is fixed to the interior of the casing 38. The magnets and the stator together form a linear motor or alternator. The Stirling machine 10 also has the conventional heat exchangers 34 and regenerator 36 that are well known to those skilled in the art. All of these components are hermetically sealed within the casing 38 that contains a pressurized working gas. There are many alternative configurations and variations as well as additional components that have been described in the prior art for Stirling machines coupled to linear electro-magnetic-mechanical transducers and that can use the present invention but they are not illustrated because they are unnecessary to a description of the invention.
As well known in the prior art, in a Stirling machine, the working gas is confined in a working space comprised of an expansion space and a compression space. The working gas is alternately expanded and compressed in order to either do work or to pump heat. The reciprocating displacer cyclically shuttles a working gas between the compression space and the expansion space which are connected in fluid communication through a heat accepter, a regenerator and a heat rejecter. The shuttling cyclically changes the relative proportion of working gas in each space. Gas that is in the expansion space, and/or gas that is flowing into the expansion space through a heat exchanger (the accepter) between the regenerator and the expansion space, accepts heat from surrounding surfaces. Gas that is in the compression space, and/or gas that is flowing into the compression space through a heat exchanger (the rejecter) between the regenerator and the compression space, rejects heat to surrounding surfaces. The gas pressure is essentially the same in both spaces at any instant of time because the spaces are interconnected through a path having a relatively low flow resistance. However, the pressure of the working gas in the work space as a whole varies cyclically and periodically. When most of the working gas is in the compression space, heat is rejected from the gas. When most of the working gas is in the expansion space, the gas accepts heat. This is true whether the machine is working as a heat pump or as an engine. The only requirement to differentiate between work produced or heat pumped, is the temperature at which the expansion process is carried out. If this expansion process temperature is higher than the temperature of the compression space, then the machine is inclined to produce work so it can function as an engine and if this expansion process temperature is lower than the compression space temperature, then the machine will pump heat from a cold source to a warm heat sink.
A Stirling machine coupled to a linear electro-magnetic-mechanical transducer is a complex oscillating system with masses reciprocating within a casing, linked by springs and damping and having various forces applied to the masses. Consequently they have natural frequencies of oscillation determined by the reciprocating masses and the springs.
The term “spring” includes mechanical springs, such as coil springs, leaf springs, planar springs, gas springs, such as a piston having a face moving in a confined volume and other springs as known in the prior art. Gas springs include the working space in a Stirling machine and, in some implementations also in the back space, apply a spring force to a moving component as the gas volume changes. As known to those in the art, generally a spring is a structure or a combination of structures that applies a force to two bodies that is proportional to the displacement of one body with respect to the other. The proportionality constant that relates the spring force to the displacement is referred to as the spring constant for the spring. A mechanical spring is sometimes referred to as being “flexed” when it is actuated or moved and changes the force it applies to the bodies to which it is connected. The same term may be applied to a gas spring in which compression or expansion of the gas spring is a flexing of the gas spring. Additionally, a spring may be a composite spring; that is, a spring having two or more component springs. For example, two springs connected in parallel to two bodies form a net or composite spring. If one of the springs is variable, that is, it has a variable spring constant, then the net or composite spring is variable. The term “spring coupling” is used to indicate that two bodies are connected by one or more springs; that is, they are coupled together by a net spring.
For purposes of describing the oscillating motion of one or more bodies, the mass of a body includes the mass of all structures that are attached to and move with it. The piston mass includes the mass of the magnets and their support structures that are attached to the piston. Similarly, the stator mass is the sum of the mass of the alternator/motor coil, low reluctance ferromagnetic core and attached mass such as mounting structures. The displacer mass includes the displacer connecting rod.
Because a Stirling machine coupled to a linear electro-magnetic-mechanical transducer has periodic, reciprocating masses, its casing 38 vibrates. Consequently, a vibration balancer 40 is commonly connected to the casing 38 to cancel the periodic vibration forces. Referring to FIG. 1, a typical vibration balancer has a plurality of masses 42 mounted to planar or leaf springs 44 or sometimes coil springs (not shown) so they too become oscillating bodies. The springs 44 are connected to the casing 38 by a connector 46. The coupled Stirling machine and linear alternator or motor has a nominal operating frequency so the vibration balancer 40 is tuned to have a natural frequency of oscillation at that operating frequency. The principle is that the balancer masses 42 and their attached springs 44 are designed so that oscillating masses 42 cause a periodic force to be applied by the springs 44 to the casing 38 with that periodic force being equal in magnitude and opposite in phase to the vibration forces applied to the casing by the reciprocating components, principally the power piston 14 and the displacer 18. In this manner, the sum of the forces applied to the casing is made equal or nearly equal to zero.